Use ConstExp for storing constants.

sdk/lib/_internal/compiler/implementation/js_emitter/container_builder.dart

Index: sdk/lib/_internal/compiler/implementation/js_emitter/container_builder.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_emitter/container_builder.dart b/sdk/lib/_internal/compiler/implementation/js_emitter/container_builder.dart
index c256b0d9d347b911faa5710df69254d494c11cfb..43d3e7b48a1a41c091a602511a1efcad8bc1349f 100644
--- a/sdk/lib/_internal/compiler/implementation/js_emitter/container_builder.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_emitter/container_builder.dart
@@ -96,10 +96,11 @@ class ContainerBuilder extends CodeEmitterHelper {
           parametersBuffer[optionalParameterStart + index] =
               new jsAst.Parameter(jsName);
         } else {
-          Constant value = handler.getConstantForVariable(element);
-          if (value == null) {
+          ConstExp constant = handler.getConstantForVariable(element);
+          if (constant == null) {
             argumentsBuffer[count] = task.constantReference(new NullConstant());
           } else {
+            Constant value = constant.value;
             if (!value.isNull) {
               // If the value is the null constant, we should not pass it
               // down to the native method.
@@ -516,7 +517,7 @@ class ContainerBuilder extends CodeEmitterHelper {
           Iterable<int> metadataIndices =
               parameter.metadata.map((MetadataAnnotation annotation) {
             Constant constant =
-                backend.constants.getConstantForMetadata(annotation);
+                backend.constants.getConstantForMetadata(annotation).value;
             backend.constants.addCompileTimeConstantForEmission(constant);
             return task.metadataEmitter.reifyMetadata(annotation);
           });