Handle casts from Number to FutureOr<double>.

pkg/dev_compiler/test/browser/runtime_tests.js

Index: pkg/dev_compiler/test/browser/runtime_tests.js
diff --git a/pkg/dev_compiler/test/browser/runtime_tests.js b/pkg/dev_compiler/test/browser/runtime_tests.js
index 3cb562aff0a9c56993f84bc34d6f72d67b77e3f2..9b13f763cafaf8bb569b74287702d6a9978a1cf2 100644
--- a/pkg/dev_compiler/test/browser/runtime_tests.js
+++ b/pkg/dev_compiler/test/browser/runtime_tests.js
@@ -132,6 +132,7 @@ define(['dart_sdk'], function(dart_sdk) {
     let List = core.List;
     let Map = core.Map;
     let Map$ = core.Map$;
+    let double = core.double;
     let int = core.int;
     let num = core.num;
     let bool = core.bool;
@@ -283,6 +284,19 @@ define(['dart_sdk'], function(dart_sdk) {
       expect(cast(null, String), null);
     });
 
+    test('FutureOr', () => {
+      let FutureOr = async.FutureOr$;
+
+      assert.equal(dart.as(3, FutureOr(int)), 3);
+      assert.equal(dart.as(3, FutureOr(double)), 3);
+      assert.throws(() => dart.as(3.5, FutureOr(int)));
+      assert.equal(dart.as(3.5, FutureOr(double)), 3.5);
+      assert.isTrue(dart.is(3, FutureOr(int)));
+      assert.isTrue(dart.is(3, FutureOr(double)));
+      assert.isFalse(dart.is(3.5, FutureOr(int)));
+      assert.isTrue(dart.is(3.5, FutureOr(double)));
+    });
+
     test('Map', () => {
       let m1 = new (Map$(String, String))();
       let m2 = new (Map$(Object, Object))();