dart2js: Match the runtime int is check semantics at compile time.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.constant_system.js;
 
 import '../compiler.dart' show
     Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
@@ skipping 256 matching lines @@
     return new ListConstantValue(type, values);
   }
 
   @override
   ConstantValue createType(Compiler compiler, DartType type) {
     return new TypeConstantValue(
         type,
         compiler.backend.typeImplementation.computeType(compiler.resolution));
   }
 
-  // Integer checks don't verify that the number is not -0.0.
-  bool isInt(ConstantValue constant) => constant.isInt || constant.isMinusZero;
+  // Integer checks report true for -0.0, INFINITY, and -INFINITY.  At
+  // runtime an 'X is int' check is implemented as:
+  //
+  // typeof(X) === "number" && Math.floor(X) === X
+  //
+  // We consistently match that runtime semantics at compile time as well.
+  bool isInt(ConstantValue constant) {
+    return constant.isInt || constant.isMinusZero ||
+        constant.isPositiveInfinity ||
+        constant.isNegativeInfinity;
+  }
   bool isDouble(ConstantValue constant)
       => constant.isDouble && !constant.isMinusZero;
   bool isString(ConstantValue constant) => constant.isString;
   bool isBool(ConstantValue constant) => constant.isBool;
   bool isNull(ConstantValue constant) => constant.isNull;
 
   bool isSubtype(DartTypes types, DartType s, DartType t) {
     // At runtime, an integer is both an integer and a double: the
     // integer type check is Math.floor, which will return true only
     // for real integers, and our double type check is 'typeof number'
@@ skipping 92 matching lines @@
       result.add(keyList);
     } else {
       // Add the keys individually to avoid generating an unused list constant
       // for the keys.
       result.addAll(keys);
     }
     result.addAll(values);
     return result;
   }
 }