Allow Observatory debugger to switch isolates.

runtime/platform/utils.h

Index: runtime/platform/utils.h
diff --git a/runtime/platform/utils.h b/runtime/platform/utils.h
index 8eff2f47062049572ef32b983763daa8f6f04343..796fbbe33a7248776b8e9df7e46bdcf7eaac0a0e 100644
--- a/runtime/platform/utils.h
+++ b/runtime/platform/utils.h
@@ -201,6 +201,19 @@ class Utils {
   static bool DoublesBitEqual(const double a, const double b) {
     return bit_cast<int64_t, double>(a) == bit_cast<int64_t, double>(b);
   }
+
+  // dart2js represents integers as double precision floats, which can
+  // represent anything in the range -2^53 ... 2^53.
+  static bool IsJavascriptInt(int64_t value) {
+    return ((-0x20000000000000LL <= value) && (value <= 0x20000000000000LL));
+  }
+  static bool IsJavascriptInt(intptr_t value) {
+#if defined(ARCH_IS_64BIT)
+    return ((-0x20000000000000LL <= value) && (value <= 0x20000000000000LL));
+#else
+    return true;
+#endif
+  }
 };
 
 }  // namespace dart