Don't rely on the existence of an element in field gets.

lib/compiler/implementation/ssa/codegen.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class SsaCodeGeneratorTask extends CompilerTask {
   final JavaScriptBackend backend;
   SsaCodeGeneratorTask(JavaScriptBackend backend)
       : this.backend = backend,
         super(backend.compiler);
   String get name() => 'SSA code generator';
@@ skipping 1827 matching lines @@
   }
 
   visitFieldGet(HFieldGet node) {
     String name =
         compiler.namer.instanceFieldName(node.library, node.fieldName);
     beginExpression(JSPrecedence.MEMBER_PRECEDENCE);
     use(node.receiver, JSPrecedence.MEMBER_PRECEDENCE);
     buffer.add('.');
     buffer.add(name);
     beginExpression(JSPrecedence.MEMBER_PRECEDENCE);
-    Type type = node.receiver.propagatedType.computeType(compiler);
-    if (type != null) {
-      world.registerFieldGetter(node.element.name, type);
+    if (node.element == null) {
+      // If we don't have an element we register a dynamic field getter.
+      // This might lead to unnecessary getters, but these cases should be
+      // rare.
+      world.registerDynamicGetter(node.fieldName, Selector.GETTER);
+    } else {
+      Type type = node.receiver.propagatedType.computeType(compiler);
+      if (type != null) {
+        world.registerFieldGetter(node.element.name, type);
+      }
     }
   }
 
   // Determine if an instruction is a simple number computation
   // involving only things with guaranteed number types and a given
   // field.
   bool isSimpleFieldNumberComputation(HInstruction value, HFieldSet node) {
     if (value.guaranteedType.union(HType.NUMBER) == HType.NUMBER) return true;
     if (value is HBinaryArithmetic) {
       return (isSimpleFieldNumberComputation(value.left, node) &&
               isSimpleFieldNumberComputation(value.right, node));
     }
     if (value is HFieldGet) return value.element == node.element;
     return false;
   }
 
   visitFieldSet(HFieldSet node) {
-    if (work.element.isGenerativeConstructorBody() &&
+    if (node.element != null &&
+        work.element.isGenerativeConstructorBody() &&
         node.element.enclosingElement.isClass() &&
         node.value.hasGuaranteedType() &&
         node.block.dominates(currentGraph.exit)) {
       backend.updateFieldConstructorSetters(node.element,
                                             node.value.guaranteedType);
     }
     String name =
         compiler.namer.instanceFieldName(node.library, node.fieldName);
     beginExpression(JSPrecedence.ASSIGNMENT_PRECEDENCE);
     use(node.receiver, JSPrecedence.MEMBER_PRECEDENCE);
     buffer.add('.');
     buffer.add(name);
-    Type type = node.receiver.propagatedType.computeType(compiler);
-    if (type != null) {
-      if (!work.element.isGenerativeConstructorBody()) {
-        world.registerFieldSetter(node.element.name, type);
-      }
-      // Determine the types seen so far for the field. If only number
-      // types have been seen and the value of the field set is a
-      // simple number computation only depending on that field, we
-      // can safely keep the number type for the field.
-      HType fieldSettersType = backend.fieldSettersTypeSoFar(node.element);
-      HType initializersType = backend.typeFromInitializersSoFar(node.element);
-      HType fieldType = fieldSettersType.union(initializersType);
-      if (HType.NUMBER.union(fieldType) == HType.NUMBER &&
-          isSimpleFieldNumberComputation(node.value, node)) {
-        backend.updateFieldSetters(node.element, HType.NUMBER);
-      } else {
-        backend.updateFieldSetters(node.element,
-                                   node.value.propagatedType);
+    if (node.element == null) {
+      // If we don't have an element we register a dynamic field setter.
+      // This might lead to unnecessary setters, but these cases should be
+      // rare.
+      world.registerDynamicSetter(node.fieldName, Selector.SETTER);
+    } else {
+      Type type = node.receiver.propagatedType.computeType(compiler);
+      if (type != null) {
+        if (!work.element.isGenerativeConstructorBody()) {
+          world.registerFieldSetter(node.element.name, type);
+        }
+        // Determine the types seen so far for the field. If only number
+        // types have been seen and the value of the field set is a
+        // simple number computation only depending on that field, we
+        // can safely keep the number type for the field.
+        HType fieldSettersType = backend.fieldSettersTypeSoFar(node.element);
+        HType initializersType =
+            backend.typeFromInitializersSoFar(node.element);
+        HType fieldType = fieldSettersType.union(initializersType);
+        if (HType.NUMBER.union(fieldType) == HType.NUMBER &&
+            isSimpleFieldNumberComputation(node.value, node)) {
+          backend.updateFieldSetters(node.element, HType.NUMBER);
+        } else {
+          backend.updateFieldSetters(node.element,
+                                     node.value.propagatedType);
+        }
       }
     }
     buffer.add(' = ');
     use(node.value, JSPrecedence.ASSIGNMENT_PRECEDENCE);
     endExpression(JSPrecedence.ASSIGNMENT_PRECEDENCE);
   }
 
   visitLocalGet(HLocalGet node) {
     use(node.receiver, JSPrecedence.EXPRESSION_PRECEDENCE);
   }
@@ skipping 1283 matching lines @@
   if (leftType.canBeNull() && rightType.canBeNull()) {
     if (left.isConstantNull() || right.isConstantNull() ||
         (leftType.isPrimitive() && leftType == rightType)) {
       return '==';
     }
     return null;
   } else {
     return '===';
   }
 }