Make test of malformed types a dynamic type error.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

Index: sdk/lib/_internal/compiler/implementation/elements/modelx.dart
diff --git a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
index 56a35e9bc2e87b07d7ae1916a9e52fb591952850..c9a420c963f053ed10d219a6ca72eabd57e5b798 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
@@ -345,9 +345,9 @@ class ErroneousElementX extends ElementX implements ErroneousElement {
 
   computeTargetType(Compiler compiler, InterfaceType newType) => unsupported();
 
-  String toString() {
-    return '<$name: ${messageKind.message(messageArguments)}>';
-  }
+  String get message => '${messageKind.message(messageArguments)}';
+
+  String toString() => '<$name: $message>';
 }
 
 /// A message attached to a [WarnOnUseElementX].