Fix two crashes.

dart/frog/leg/ssa/builder.dart

 // Copyright (c) 2011, 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 Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 1037 matching lines @@
         visitStaticSend(node);
       } else {
         compiler.internalError("Cannot generate code for send", node: node);
       }
     }
   }
 
   visitNewExpression(NewExpression node) => visitSend(node.send);
 
   HInstruction updateDefinition(Node node, HInstruction value) {
-    VariableElement element = elements[node];
+    Element element = elements[node];
+    if (element is !VariableElement) {
+      compiler.internalError("expected a variable", node: node);
+    }
     definitions[element] = value;
     return value;
   }
 
   visitSendSet(SendSet node) {
     Operator op = node.assignmentOperator;
     if (node.isIndex) {
       HStatic target = new HStatic(interceptors.getIndexAssignmentInterceptor());
       add(target);
       visit(node.receiver);
@@ skipping 345 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     compiler.unimplemented('SsaBuilder.visitCatchBlock', node: node);
   }
 
   visitTypedef(Typedef node) {
     compiler.unimplemented('SsaBuilder.visitTypedef', node: node);
   }
 }