Implement fromEnvironment on bool, int and String

sdk/lib/core/int.dart

Index: sdk/lib/core/int.dart
diff --git a/sdk/lib/core/int.dart b/sdk/lib/core/int.dart
index a5235bbd66b878001f36a7bf52fcdd6e277e4194..1c17a317b39002935fe4c7d65be09445ce2309d4 100644
--- a/sdk/lib/core/int.dart
+++ b/sdk/lib/core/int.dart
@@ -252,4 +252,12 @@ abstract class int extends num {
   external static int parse(String source,
                             { int radix,
                               int onError(String source) });
+
+  /**
+   * Returns the integer value for the given environment variable
+   * [name] or [defaultValue] if [name] is not present. If the value
+   * of the environment variable is not a valid integer literal a
+   * [FormatException] is thrown.
+   */
+  external const factory int.fromEnvironment(String name, {int defaultValue});
 }