Support patched generic methods.

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

Index: pkg/compiler/lib/src/js_backend/patch_resolver.dart
diff --git a/pkg/compiler/lib/src/js_backend/patch_resolver.dart b/pkg/compiler/lib/src/js_backend/patch_resolver.dart
index 17a4a4e33edba1d436a1fad8c427aafefde7e5e8..10f30c581ac22bf5b67070ff98afe66702a36296 100644
--- a/pkg/compiler/lib/src/js_backend/patch_resolver.dart
+++ b/pkg/compiler/lib/src/js_backend/patch_resolver.dart
@@ -106,10 +106,24 @@ class PatchResolverTask extends CompilerTask {
   void checkMatchingPatchSignatures(
       FunctionElement origin, FunctionElement patch) {
     // TODO(johnniwinther): Show both origin and patch locations on errors.
+    FunctionExpression originTree = origin.node;
     FunctionSignature originSignature = origin.functionSignature;
     FunctionExpression patchTree = patch.node;
     FunctionSignature patchSignature = patch.functionSignature;
 
+    if ('${originTree.typeVariables}' != '${patchTree.typeVariables}') {

[Siggi Cherem (dart-lang), 2016/11/29 18:06:56]

do we use .toString anywhere else for this kind of comparison? Could we compare
it programmatically instead?

Also, would we fail if you use different variable names, but that are equivalent
otherwise? 

[Johnni Winther, 2016/11/30 08:14:29]

We also do it in [checkMatchingPatchParameters] to ensure matching default
values. We don't need to support variance to this is the easiest way to guard
against accidental mismatch.
We would and should since the body would be resolved in a scope with the
original type variables. Test added.

+      reporter.withCurrentElement(patch, () {
+        Node errorNode = patchTree.typeVariables != null
+            ? patchTree.typeVariables
+            : patchTree;
+        reporter.reportError(
+            reporter.createMessage(
+                errorNode,
+                MessageKind.PATCH_TYPE_VARIABLES_MISMATCH,
+                {'methodName': origin.name}),
+            [reporter.createMessage(origin, MessageKind.THIS_IS_THE_METHOD)]);
+      });
+    }
     if (originSignature.type.returnType != patchSignature.type.returnType) {
       reporter.withCurrentElement(patch, () {
         Node errorNode =