Add optional start index to Pattern.allMatches.

sdk/lib/_internal/lib/js_string.dart

Index: sdk/lib/_internal/lib/js_string.dart
diff --git a/sdk/lib/_internal/lib/js_string.dart b/sdk/lib/_internal/lib/js_string.dart
index 36372f46c9caee94754fe3ae48935c9a4d676443..a6962cb2432eb24af6e5a6f614d75bbdd3014bc9 100644
--- a/sdk/lib/_internal/lib/js_string.dart
+++ b/sdk/lib/_internal/lib/js_string.dart
@@ -20,9 +20,13 @@ class JSString extends Interceptor implements String, JSIndexable {
     return JS('JSUInt31', r'#.charCodeAt(#)', this, index);
   }
 
-  Iterable<Match> allMatches(String str) {
-    checkString(str);
-    return allMatchesInStringUnchecked(this, str);
+  Iterable<Match> allMatches(String string, [int start = 0]) {
+    checkString(string);
+    checkInt(start);
+    if (0 > start || start > string.length) {
+      throw new RangeError.range(start, 0, string.length);
+    }
+    return allMatchesInStringUnchecked(this, string, start);
   }
 
   Match matchAsPrefix(String string, [int start = 0]) {