Allow use of deferred type-literals in non-constant contexts.

pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart

Index: pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart
diff --git a/pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart b/pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart
index 4771a5bc6e44f27c3d9370cb97b5a89a27d675bb..0f66c0d7a968e3328caa821e80ee2f22d67a9600 100644
--- a/pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart
@@ -41,11 +41,14 @@ class JavaScriptConstantTask extends ConstantCompilerTask {
     });
   }
 
-  ConstantExpression compileNode(Node node, TreeElements elements) {
+  ConstantExpression compileNode(Node node, TreeElements elements,
+                                 {bool enforceConst: true}) {
     return measure(() {
       ConstantExpression result =
-          dartConstantCompiler.compileNode(node, elements);
-      jsConstantCompiler.compileNode(node, elements);
+          dartConstantCompiler.compileNode(node, elements,
+                                           enforceConst: enforceConst);
+      jsConstantCompiler.compileNode(node, elements,
+          enforceConst: enforceConst);
       return result;
     });
   }
@@ -163,8 +166,9 @@ class JavaScriptConstantCompiler extends ConstantCompilerBase
     return initialValue;
   }
 
-  ConstantExpression compileNode(Node node, TreeElements elements) {
-    return compileNodeWithDefinitions(node, elements);
+  ConstantExpression compileNode(Node node, TreeElements elements,
+                                 {bool enforceConst: true}) {
+    return compileNodeWithDefinitions(node, elements, isConst: enforceConst);
   }
 
   ConstantExpression compileNodeWithDefinitions(Node node,