Skip spurious constants in deferred computation.

pkg/compiler/lib/src/compile_time_constants.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.
 
 library dart2js.compile_time_constant_evaluator;
 
 import 'common.dart';
 import 'common/resolution.dart' show Resolution;
 import 'common/tasks.dart' show CompilerTask;
 import 'compiler.dart' show Compiler;
@@ skipping 11 matching lines @@
 import 'tree/tree.dart';
 import 'util/util.dart' show Link;
 import 'universe/call_structure.dart' show CallStructure;
 
 /// A [ConstantEnvironment] provides access for constants compiled for variable
 /// initializers.
 abstract class ConstantEnvironment {
   /// The [ConstantSystem] used by this environment.
   ConstantSystem get constantSystem;
 
+  /// Returns `true` if a value has been computed for [expression].
+  bool hasConstantValue(ConstantExpression expression);
+
   /// Returns the constant value computed for [expression].
   // TODO(johnniwinther): Support directly evaluation of [expression].
   ConstantValue getConstantValue(ConstantExpression expression);
 
   /// Returns the constant value for the initializer of [element].
   @deprecated
   ConstantValue getConstantValueForVariable(VariableElement element);
 }
 
 /// A class that can compile and provide constants for variables, nodes and
@@ skipping 238 matching lines @@
         this, definitions, compiler,
         isConst: isConst);
     AstConstant constant = evaluator.evaluate(node);
     if (constant != null) {
       cacheConstantValue(constant.expression, constant.value);
       return constant.expression;
     }
     return null;
   }
 
+  bool hasConstantValue(ConstantExpression expression) {
+    return constantValueMap.containsKey(expression);
+  }
+
   ConstantValue getConstantValue(ConstantExpression expression) {
     return constantValueMap[expression];
   }
 
   ConstantExpression compileNode(Node node, TreeElements elements,
       {bool enforceConst: true}) {
     return compileNodeWithDefinitions(node, elements, isConst: enforceConst);
   }
 
   ConstantExpression compileMetadata(
@@ skipping 1035 matching lines @@
 class _CompilerEnvironment implements Environment {
   final Compiler compiler;
 
   _CompilerEnvironment(this.compiler);
 
   @override
   String readFromEnvironment(String name) {
     return compiler.fromEnvironment(name);
   }
 }