Port and integrate the irregexp engine from V8

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 3a1fd711b6cd06e4a46f44ad383d6258a8d47a76..a65a6bc26257ffe2311814041b4893e739f348cc 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -739,6 +739,27 @@ class _OneByteString extends _StringBase implements String {
     return super.split(pattern);
   }
 
+  // The codeUnitsAt family of functions is inlined to an unchecked
+  // LoadCodeUnitsInstr, therefore `this` must be a valid string, and `index`
+  // must be in bounds.
+  int _oneCodeUnitAt(int index) => _codeUnitsAt(index, 1);
+  int _twoCodeUnitsAt(int index) => _codeUnitsAt(index, 2);
+  int _fourCodeUnitsAt(int index) => _codeUnitsAt(index, 4);
+
+  // Loads up to 4 code units into a single integer.
+  int _codeUnitsAt(int index, int count) {
+    assert(index >= 0);
+    assert(index + count - 1 < this.length);
+    assert(0 < count && count <= 4);
+
+    int codeUnits = 0;
+    for (int i = 0; i < count; i++) {
+      codeUnits |= this.codeUnitAt(index + i) << (i * 8);
+    }
+
+    return codeUnits;
+  }
+
   // All element of 'strings' must be OneByteStrings.
   static _concatAll(List<String> strings, int totalLength) {
     // TODO(srdjan): Improve code below and raise or eliminate the limit.
@@ -1003,6 +1024,23 @@ class _TwoByteString extends _StringBase implements String {
   bool operator ==(Object other) {
     return super == other;
   }
+
+  int _oneCodeUnitAt(int index) => _codeUnitsAt(index, 1);
+  int _twoCodeUnitsAt(int index) => _codeUnitsAt(index, 2);
+
+  // Loads up to 2 code units into a single integer.
+  int _codeUnitsAt(int index, int count) {
+    assert(index >= 0);
+    assert(index + count - 1 < this.length);
+    assert(0 < count && count <= 2);
+
+    int codeUnits = 0;
+    for (int i = 0; i < count; i++) {
+      codeUnits |= this.codeUnitAt(index + i) << (i * 16);
+    }
+
+    return codeUnits;
+  }
 }