Proof-of-concept matchPrefix on Pattern.

tests/corelib/string_pattern_test.dart

Index: tests/corelib/string_pattern_test.dart
diff --git a/tests/corelib/string_pattern_test.dart b/tests/corelib/string_pattern_test.dart
index 46b4e75d59ed1a9e60376311c5a30d47ad675c18..25e12e795f9add7ed9f6d056c5a979fd93b10ab6 100644
--- a/tests/corelib/string_pattern_test.dart
+++ b/tests/corelib/string_pattern_test.dart
@@ -14,6 +14,7 @@ main() {
   testEmptyPattern();
   testEmptyString();
   testEmptyPatternAndString();
+  testMatchAsPrefix();
 }
 
 testNoMatch() {
@@ -77,3 +78,22 @@ testEmptyPatternAndString() {
   Iterable<Match> matches = pattern.allMatches(str);
   Expect.isTrue(matches.iterator.moveNext());
 }
+
+testMatchAsPrefix() {
+  String pattern = "an";
+  String str = "banana";
+  Expect.isNull(pattern.matchAsPrefix(str));
+  Expect.isNull(pattern.matchAsPrefix(str, 0));
+  var m = pattern.matchAsPrefix(str, 1);
+  Expect.equals("an", m[0]);
+  Expect.equals(1, m.start);
+  Expect.isNull(pattern.matchAsPrefix(str, 2));
+  m = pattern.matchAsPrefix(str, 3);
+  Expect.equals("an", m[0]);
+  Expect.equals(3, m.start);
+  Expect.isNull(pattern.matchAsPrefix(str, 4));
+  Expect.isNull(pattern.matchAsPrefix(str, 5));
+  Expect.isNull(pattern.matchAsPrefix(str, 6));
+  Expect.throws(() => pattern.matchAsPrefix(str, -1));
+  Expect.throws(() => pattern.matchAsPrefix(str, 7));
+}