Fix invariant breakage on erroneous constant variables.

pkg/compiler/lib/src/elements/modelx.dart

 // Copyright (c) 2013, 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.
 
 library elements.modelx;
 
 import '../common.dart';
 import '../common/resolution.dart' show Resolution, ParsingContext;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_constructors.dart';
@@ skipping 1390 matching lines @@
         message: "Constant has not been computed for $this."));
     return constantCache;
   }
 
   void set constant(ConstantExpression value) {
     if (isPatch) {
       ConstantVariableMixin originVariable = origin;
       originVariable.constant = value;
       return;
     }
+    if (constantCache != null &&
+        constantCache.kind == ConstantExpressionKind.ERRONEOUS) {
+      // TODO(johnniwinther): Find out why we sometimes compute a non-erroneous
+      // constant for a variable already known to be erroneous.
+      return;
+    }
     assert(invariant(this, constantCache == null || constantCache == value,
         message: "Constant has already been computed for $this. "
             "Existing constant: "
-            "${constantCache != null ? constantCache.toDartText() : ''}, "
-            "New constant: ${value != null ? value.toDartText() : ''}."));
+            "${constantCache != null ? constantCache.toStructuredText() : ''}, "
+            "New constant: ${value != null ? value.toStructuredText() : ''}."));
     constantCache = value;
   }
 }
 
 abstract class VariableElementX extends ElementX
     with AstElementMixin, ConstantVariableMixin
     implements VariableElement {
   final Token token;
   final VariableList variables;
   VariableDefinitions definitionsCache;
@@ skipping 1848 matching lines @@
     if (definingElement.isField) {
       FieldElement field = definingElement;
       body = field.initializer;
     } else if (node != null && node.asFunctionExpression() != null) {
       body = node.asFunctionExpression().body;
     }
     return new ParsedResolvedAst(
         declaration, node, body, definingElement.treeElements);
   }
 }