Fix ParameterMirror.hasDefaultValue/defaultValue in source mirrors.

sdk/lib/_internal/compiler/implementation/mirrors/dart2js_member_mirrors.dart

Index: sdk/lib/_internal/compiler/implementation/mirrors/dart2js_member_mirrors.dart
diff --git a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_member_mirrors.dart b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_member_mirrors.dart
index 3f4b60489c1cf3634d41a210fcd8727d12e955d0..0a22d4d8724119bdb6004c9d1c428bf8ea4313f9 100644
--- a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_member_mirrors.dart
+++ b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_member_mirrors.dart
@@ -239,17 +239,19 @@ class Dart2JsParameterMirror extends Dart2JsMemberMirror
   bool get isConst => false;
 
   InstanceMirror get defaultValue {
-    Constant constant = mirrorSystem.compiler.constantHandler
-        .getConstantForVariable(_variableElement);
-    if (constant != null) {
+    if (hasDefaultValue) {
+      Constant constant = mirrorSystem.compiler.constantHandler
+          .getConstantForVariable(_variableElement);
+      assert(invariant(_variableElement, constant != null,
+          message: "Missing constant for parameter "
+                   "$_variableElement with default value."));
       return _convertConstantToInstanceMirror(mirrorSystem, constant);
     }
     return null;
   }
 
   bool get hasDefaultValue {
-    return mirrorSystem.compiler.constantHandler
-        .getConstantForVariable(_variableElement) != null;
+    return _variableElement.cachedNode.asSendSet() != null;
   }
 
   bool get isInitializingFormal => false;