Throw Error on unreachable code rather than a String.

sdk/lib/_internal/js_runtime/lib/js_rti.dart

Index: sdk/lib/_internal/js_runtime/lib/js_rti.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_rti.dart b/sdk/lib/_internal/js_runtime/lib/js_rti.dart
index ae8fc9d72f088c26b54579842fcaf22e57995f4d..60af58d2331934fe80ff2803951787b68ab29902 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_rti.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_rti.dart
@@ -41,7 +41,11 @@
 
 part of _js_helper;
 
-Type createRuntimeType(String name) => new TypeImpl(name);
+Type createRuntimeType(String name) {
+  // Use a 'JS' cast to String.  Since this is registered as used by the
+  // backend, type inference assumes the worst (name is dynamic).
+  return new TypeImpl(JS('String', '#', name));
+}
 
 class TypeImpl implements Type {
   final String _typeName;