Proof-of-concept matchPrefix on Pattern.

runtime/lib/string_patch.dart

Index: runtime/lib/string_patch.dart
diff --git a/runtime/lib/string_patch.dart b/runtime/lib/string_patch.dart
index 16c3232c4a0ba1c02ac0b597c7c07314ba7f9762..4612571698742625156e02be247ccb164c825241 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -421,6 +421,19 @@ class _StringBase {
     return result;
   }
 
+  Match matchAsPrefix(String string, [int start = 0]) {
+    if (start < 0 || start > string.length) {
+      throw new RangeError.range(start, 0, string.length);
+    }
+    if (start + this.length > string.length) return null;
+    for (int i = 0; i < this.length; i++) {
+      if (string.codeUnitAt(start + i) != this.codeUnitAt(i)) {
+        return null;
+      }
+    }
+    return new _StringMatch(start, string, this);
+  }
+
   List<String> split(Pattern pattern) {
     if ((pattern is String) && pattern.isEmpty) {
       List<String> result = new List<String>(length);