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

lib/compiler/implementation/ssa/optimize.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.
 
 interface OptimizationPhase {
   String get name();
   void visitGraph(HGraph graph);
 }
 
 class SsaOptimizerTask extends CompilerTask {
@@ skipping 1191 matching lines @@
       HType type = HType.NUMBER.union(fieldType);
       if (type == HType.NUMBER) {
         handleFieldNumberOperation(field, fieldType);
       }
     }
   }
 
   void visitFieldGet(HFieldGet node) {
     if (!node.element.isInstanceMember()) return;
     Element field = node.element;
-    HType type = backend.optimisticFieldTypeAfterConstruction(field);
-    if (!type.isUnknown()) {
-      // Allow handling even if we haven't seen any types for this
-      // field yet. There might still be only one setter in an
-      // initializer list or constructor body and recompilation
-      // can therefore pay off.
-      handleFieldGet(node, type);
+    if (field != null) {
+      HType type = backend.optimisticFieldTypeAfterConstruction(field);
+      if (!type.isUnknown()) {
+        // Allow handling even if we haven't seen any types for this
+        // field yet. There might still be only one setter in an
+        // initializer list or constructor body and recompilation
+        // can therefore pay off.
+        handleFieldGet(node, type);
+      }
     }
   }
 
   HInstruction visitEquals(HEquals node) {
     checkFieldNumberOperation(node);
   }
 
   HInstruction visitBinaryArithmetic(HBinaryArithmetic node) {
     checkFieldNumberOperation(node);
   }
@@ skipping 74 matching lines @@
       // this type for the field is still a strong signal
       // indicating the expected type of the field.
       field.propagatedType = type;
     } else {
       // If there are no invoked setters we know the type of
       // this field for sure.
       field.guaranteedType = type;
     }
   }
 }