Implement more missing pieces in VM's core library. Although it does not seem to make sense to have…

runtime/lib/integers.dart

Index: runtime/lib/integers.dart
===================================================================
--- runtime/lib/integers.dart	(revision 4458)
+++ runtime/lib/integers.dart	(working copy)
@@ -127,16 +127,13 @@
   }
 
   String toStringAsFixed(int fractionDigits) {
-    // Issue 460.
-    throw "IntegerImplementation.toStringAsFixed not implemented";
+    return this.toDouble().toStringAsFixed(fractionDigits);
   }
   String toStringAsExponential(int fractionDigits) {
-    // Issue 460.
-    throw "IntegerImplementation.toStringAsExponential not implemented";
+    return this.toDouble().toStringAsExponential(fractionDigits);
   }
   String toStringAsPrecision(int precision) {
-    // Issue 460.
-    throw "IntegerImplementation.toStringAsPrecision not implemented";
+    return this.toDouble().toStringAsPrecision(precision);
   }
   String toRadixString(int radix) {
     final table = const ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9",