implement generic method runtime behavior, fixes #301

test/codegen/expect/unittest.js

 dart_library.library('unittest', null, /* Imports */[
   'dart_sdk',
   'matcher'
 ], function(exports, dart_sdk, matcher) {
   'use strict';
   const core = dart_sdk.core;
   const async = dart_sdk.async;
   const js = dart_sdk.js;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
@@ skipping 21 matching lines @@
   unittest.Throws = class Throws extends src__interfaces.Matcher {
     Throws(matcher) {
       if (matcher === void 0) matcher = null;
       this[_matcher] = matcher;
       super.Matcher();
     }
     matches(item, matchState) {
       if (!dart.is(item, core.Function) && !dart.is(item, async.Future)) return false;
       if (dart.is(item, async.Future)) {
         let done = dart.dcall(unittest._wrapAsync, dart.fn(fn => dart.dcall(fn)));
-        item.then(dart.fn(value => {
+        item.then(dart.dynamic)(dart.fn(value => {
           dart.dcall(done, dart.fn(() => {
             unittest.fail(`Expected future to fail, but succeeded with '${value}'.`);
           }));
         }), {onError: dart.fn((error, trace) => {
             dart.dcall(done, dart.fn(() => {
               if (this[_matcher] == null) return;
               let reason = null;
               if (trace != null) {
                 let stackTrace = dart.toString(trace);
                 stackTrace = `  ${stackTrace[dartx.replaceAll]("\n", "\n  ")}`;
@@ skipping 57 matching lines @@
   dart.fn(unittest.group, dart.void, [core.String, dart.functionType(dart.void, [])]);
   unittest.test = function(name, body, opts) {
     let skip = opts && 'skip' in opts ? opts.skip : null;
     if (skip != null) {
       core.print(`SKIP ${name}: ${skip}`);
       return;
     }
     let result = dart.as(js.context.callMethod('test', dart.list([name, dart.fn(done => {
         function _finishTest(f) {
           if (dart.is(f, async.Future)) {
-            f.then(_finishTest);
+            f.then(dart.dynamic)(_finishTest);
           } else {
             done.apply([]);
           }
         }
         dart.fn(_finishTest);
         _finishTest(body());
       }, dart.dynamic, [js.JsFunction])], core.Object)), js.JsObject);
     result.set('async', 1);
   };
   dart.fn(unittest.test, dart.void, [core.String, dart.functionType(dart.dynamic, [])], {skip: core.String});
@@ skipping 122 matching lines @@
   unittest.greaterThan = src__numeric_matchers.greaterThan;
   unittest.isNonNegative = src__numeric_matchers.isNonNegative;
   unittest.isNull = src__core_matchers.isNull;
   unittest.isMap = src__core_matchers.isMap;
   unittest.stringContainsInOrder = src__string_matchers.stringContainsInOrder;
   unittest.closeTo = src__numeric_matchers.closeTo;
   unittest.Description = src__interfaces.Description;
   // Exports:
   exports.unittest = unittest;
 });