Move source maps package to the dart repo, so it can be used by other internal

pkg/source_maps/lib/src/utils.dart

+// Copyright (c) 2013, 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.
+
+/// Utilities that shouldn't be in this package.
+library source_maps.utils;
+
+/// Find the first entry in a sorted [list] that matches a monotonic predicate.
+/// Given a result `n`, that all items before `n` will not match, `n` matches,
+/// and all items after `n` match too. The result is -1 when there are no
+/// items, 0 when all items match, and list.length when none does.
+// TODO(sigmund): remove this function after dartbug.com/5624 is fixed.
+int binarySearch(List list, bool matches(item)) {
+  if (list.length == 0) return -1;
+  if (matches(list.first)) return 0;
+  if (!matches(list.last)) return list.length;
+
+  int min = 0;
+  int max = list.length - 1;
+  while (min < max) {
+    var half = min + ((max - min) ~/ 2);
+    if (matches(list[half])) {
+      max = half;
+    } else {
+      min = half + 1;
+    }
+  }
+  return max;
+}