Make LineSplitter much faster for common case

sdk/lib/convert/line_splitter.dart

Index: sdk/lib/convert/line_splitter.dart
diff --git a/sdk/lib/convert/line_splitter.dart b/sdk/lib/convert/line_splitter.dart
index b5ec32bd6ee94bce6e248a2913cbf5c2e970991a..b9023f4c8fb246bfa235ef8c9e20ffa8120ac9c1 100644
--- a/sdk/lib/convert/line_splitter.dart
+++ b/sdk/lib/convert/line_splitter.dart
@@ -49,7 +49,29 @@ class LineSplitter extends Converter<String, List<String>> {
     }
   }
 
-  List<String> convert(String data) => split(data).toList();
+  List<String> convert(String data) {
+    List<String> lines = <String>[];
+    int end = data.length;
+    int sliceStart = 0;
+    int char = 0;
+    for (int i = 0; i < end; i++) {
+      int previousChar = char;
+      char = data.codeUnitAt(i);
+      if (char != _CR) {
+        if (char != _LF) continue;
+        if (previousChar == _CR) {
+          sliceStart = i + 1;
+          continue;
+        }
+      }
+      lines.add(data.substring(sliceStart, i));
+      sliceStart = i + 1;
+    }
+    if (sliceStart < end) {
+      lines.add(data.substring(sliceStart, end));
+    }
+    return lines;
+  }
 
   StringConversionSink startChunkedConversion(Sink<String> sink) {
     if (sink is! StringConversionSink) {