Proof-of-concept matchPrefix on Pattern.

sdk/lib/core/pattern.dart

Index: sdk/lib/core/pattern.dart
diff --git a/sdk/lib/core/pattern.dart b/sdk/lib/core/pattern.dart
index ac7815dd6ec1fc6245318a21d8ea9f973c4a582f..d85c8ac970f9102f3d8fcf086e687fa4a353adc5 100644
--- a/sdk/lib/core/pattern.dart
+++ b/sdk/lib/core/pattern.dart
@@ -5,5 +5,28 @@
 part of dart.core;
 
 abstract class Pattern {
+  /**
+   * Match this pattern against the string repeatedly.
+   *
+   * The iterable will contain all the non-overlapping matches of the
+   * pattern on the string, ordered by start index.
+   *
+   * The matches are found by repeatedly finding the first match
+   * of the pattern on the string, starting from the end of the previous
+   * match, and initially starting from index zero.
+   *
+   * If the pattern matches the empty string at some point, the next
+   * match is found by starting at the previous match's end plus one.
+   */
   Iterable<Match> allMatches(String str);
+
+  /**
+   * Match this pattern against the start of string.
+   *
+   * If [start] is provided, it must be an integer in the range `0` ..
+   * `string.length`. In that case, this patten is tested against the
+   * string at the [start] position. That is, a match is returned if the
+   * pattern can match a part of the string starting from position [start].
+   */
+  Match matchAsPrefix(String string, [int start = 0]);
 }