Add num.parse.

sdk/lib/core/num.dart

Index: sdk/lib/core/num.dart
diff --git a/sdk/lib/core/num.dart b/sdk/lib/core/num.dart
index 674ab2a469f22d96f265a20fdece0016e3bc52a7..c6bb4f85288833b2f71cf606633eb773d0d1b702 100644
--- a/sdk/lib/core/num.dart
+++ b/sdk/lib/core/num.dart
@@ -359,4 +359,33 @@ abstract class num implements Comparable<num> {
    *
    */
   String toString();
+
+  /**
+   * Parses a string containing a number literal into a number.
+   *
+   * The method first tries to read the [input] as integer (similar to
+   * [int.parse] without a radix).
+   * If that fails, it tries to parse the [input] as a double (similar to
+   * [double.parse]).
+   * If that fails, too, it invokes [onError] with [input].
+   *
+   * If no [onError] is supplied, it defaults to a function that throws a
+   * [FormatException].
+   *
+   * For any number `n`, this function satisfies
+   * `identical(n, num.parse(n.toString()))`.
+   */
+  static num parse(String input, [num onError(String input)]) {
+    String source = input.trim();
+    // TODO(lrn): Optimize to detect format and result type in one check.
+    num result = int.parse(source, onError: _returnNull);
+    if (result != null) return result;
+    result = double.parse(source, _returnNull);
+    if (result != null) return result;
+    if (onError == null) throw new FormatException(input);
+    return onError(input);
+  }
+
+  /** Helper function for [parse]. */
+  static _returnNull(_) => null;
 }