Skip spurious constants in deferred computation.

pkg/compiler/lib/src/compile_time_constants.dart

Index: pkg/compiler/lib/src/compile_time_constants.dart
diff --git a/pkg/compiler/lib/src/compile_time_constants.dart b/pkg/compiler/lib/src/compile_time_constants.dart
index 629d4570d32ab647f50078a36a7ec727c05da9b5..724e6f669eca41b3b3742ed62d178f622b6ba15a 100644
--- a/pkg/compiler/lib/src/compile_time_constants.dart
+++ b/pkg/compiler/lib/src/compile_time_constants.dart
@@ -29,6 +29,9 @@ 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);
@@ -287,6 +290,10 @@ abstract class ConstantCompilerBase implements ConstantCompiler {
     return null;
   }
 
+  bool hasConstantValue(ConstantExpression expression) {
+    return constantValueMap.containsKey(expression);
+  }
+
   ConstantValue getConstantValue(ConstantExpression expression) {
     return constantValueMap[expression];
   }