Proof-of-concept matchPrefix on Pattern.

sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart

Index: sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart b/sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart
index 45ab03c85aa57d738c66c9740c750559c7926247..d2feaa81ee5643a83bdcfab971d401199ca6684d 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart
@@ -33,6 +33,12 @@ class JSSyntaxRegExp implements RegExp {
                        other.isCaseSensitive,
                        true);
 
+  JSSyntaxRegExp._anchoredVersionOf(JSSyntaxRegExp other)
+      : this._internal(other.pattern + "|()",
+                       other.isMultiLine,
+                       other.isCaseSensitive,
+                       true);
+
   static makeNative(
       String pattern, bool multiLine, bool caseSensitive, bool global) {
     checkString(pattern);
@@ -82,6 +88,26 @@ class JSSyntaxRegExp implements RegExp {
     return new _AllMatchesIterable(this, str);
   }
 
+  Match matchAsPrefix(String string, [int start = 0]) {
+    if (start < 0 || start > string.length) {
+      throw new RangeError.range(start, 0, string.length);
+    }
+    // An "anchored version" of a regexp is created by adding "|()" to the
+    // source. This means that the regexp always matches at the first position
+    // that it tries, and you can see if the original regexp matched, or it
+    // was the added zero-width match that matched, by looking at the last
+    // capture. If it is a String, the match participated, otherwise it didn't.
+    JSSyntaxRegExp regexp = new JSSyntaxRegExp._anchoredVersionOf(this);
+    if (start > 0) {
+      JS("void", "#.lastIndex = #", regExpGetNative(regexp), start);
+    }
+    _MatchImplementation match = regexp.firstMatch(string);
+    if (match == null) return null;
+    if (match._groups[match._groups.length - 1] != null) return null;
+    match._groups.length -= 1;
+    return match;
+  }
+
   String get pattern => _pattern;
   bool get isMultiLine => _isMultiLine;
   bool get isCaseSensitive => _isCaseSensitive;