Implement List.getRange.

runtime/lib/arrays.dart

Index: runtime/lib/arrays.dart
===================================================================
--- runtime/lib/arrays.dart	(revision 402)
+++ runtime/lib/arrays.dart	(working copy)
@@ -86,5 +86,17 @@
     }
     return -1;
   }
+
+  static void rangeCheck(Array a, int start, int length) {
+    if (length < 0) {
+      throw new IllegalArgumentException("negative length $length");
+    }
+    if (start < 0 || start > a.length) {
+      throw new IndexOutOfRangeException(start);
+    }
+    if (start + length > a.length) {
+      throw new IndexOutOfRangeException(start + length);
+    }
+  }
 }