First shot at source maps generation in dart2js.

lib/compiler/implementation/source_map_builder.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#library('source_map_builder');
+
+#import('dart:json');
+
+#import('source_file.dart');
+
+class SourceMappingEntry {
+  SourceFile sourceFile;
+  int sourceOffset;
+  int targetOffset;
+  String sourceName;
+
+  SourceMappingEntry(this.sourceFile,
+                     this.sourceOffset,
+                     this.targetOffset,
+                     [this.sourceName]);
+}
+
+class SourceMapBuilder {
+  static final int VLQ_BASE_SHIFT = 5;
+  static final int VLQ_BASE_MASK = (1 << 5) - 1;
+  static final int VLQ_CONTINUATION_BIT = 1 << 5;
+  static final int VLQ_CONTINUATION_MASK = 1 << 5;
+  static final String BASE64_DIGITS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmn'
+                                      'opqrstuvwxyz0123456789+/';
+
+  Map<String, int> sourceURLMap;

[ahe, 2012/06/21 08:12:13]

Forgot to mention this earlier: we try to use camelCase even for acronyms. So it
should be "sourceUrlMap".

[podivilov, 2012/06/21 09:50:42]

Done.

+  List<String> sourceURLList;
+  Map<String, int> sourceNameMap;
+  List<String> sourceNameList;
+
+  int previousTargetLine;
+  int previousTargetColumn;
+  int previousSourceURLIndex;
+  int previousSourceLine;
+  int previousSourceColumn;
+  int previousSourceNameIndex;
+  bool firstEntryInLine;
+
+  SourceMapBuilder() {
+    sourceURLMap = new Map<String, int>();
+    sourceURLList = new List<String>();
+    sourceNameMap = new Map<String, int>();
+    sourceNameList = new List<String>();
+
+    previousTargetLine = 0;
+    previousTargetColumn = 0;
+    previousSourceURLIndex = 0;
+    previousSourceLine = 0;
+    previousSourceColumn = 0;
+    previousSourceNameIndex = 0;
+    firstEntryInLine = true;
+  }
+
+  String build(List<SourceMappingEntry> mappingEntries, SourceFile targetFile) {
+    StringBuffer buffer = new StringBuffer();
+    buffer.add('{\n');
+    buffer.add('  "version": 3,\n');
+    buffer.add('  "mappings": "');
+    mappingEntries.forEach((SourceMappingEntry entry) {
+      writeEntry(entry, targetFile, buffer);
+    });
+    buffer.add('",\n');
+    buffer.add('  "sources": ${JSON.stringify(sourceURLList)},\n');
+    buffer.add('  "names": ${JSON.stringify(sourceNameList)}\n');
+    buffer.add('}\n');
+    return buffer.toString();

[ahe, 2012/06/21 08:12:13]

Let's say sourceURLList contains the string "http://example.com/". How many
times is that string copied because you use JSON.stringify and String
interpolation above?

[podivilov, 2012/06/21 09:50:42]

Added TODO to migrate to fast version when corresponding API is available.

+  }
+
+  void writeEntry(SourceMappingEntry entry,
+                  SourceFile targetFile,
+                  StringBuffer output) {
+    int targetLine = targetFile.getLine(entry.targetOffset);
+    int targetColumn = targetFile.getColumn(targetLine, entry.targetOffset);
+    String sourceURL = entry.sourceFile.filename;
+    int sourceLine = entry.sourceFile.getLine(entry.sourceOffset);
+    int sourceColumn = entry.sourceFile.getColumn(sourceLine,
+                                                  entry.sourceOffset);
+    String sourceName = entry.sourceName;
+
+    if (targetLine > previousTargetLine) {
+      for (int i = previousTargetLine; i < targetLine; ++i) {
+        output.add(';');
+      }
+      previousTargetLine = targetLine;
+      previousTargetColumn = 0;
+      firstEntryInLine = true;
+    }
+
+    if (!firstEntryInLine) {
+      output.add(',');
+    }
+    firstEntryInLine = false;
+
+    encodeVLQ(output, targetColumn - previousTargetColumn);
+    previousTargetColumn = targetColumn;
+
+    if (sourceURL === null)
+      return;
+
+    int sourceURLIndex = indexOf(sourceURLList, sourceURL, sourceURLMap);
+    encodeVLQ(output, sourceURLIndex - previousSourceURLIndex);
+    previousSourceURLIndex = sourceURLIndex;
+
+    encodeVLQ(output, sourceLine - previousSourceLine);
+    previousSourceLine = sourceLine;
+    encodeVLQ(output, sourceColumn - previousSourceColumn);
+    previousSourceColumn = sourceColumn;
+
+    if (sourceName === null)
+      return;
+
+    int sourceNameIndex = indexOf(sourceNameList, sourceName, sourceNameMap);
+    encodeVLQ(output, sourceNameIndex - previousSourceNameIndex);
+    previousSourceNameIndex = sourceNameIndex;
+  }
+
+  int indexOf(List<String> list, String value, Map<String, int> map) {
+    return map.putIfAbsent(value, () {
+      int index = list.length;
+      map[value] = index;
+      list.add(value);
+      return index;
+    });
+  }
+
+  static void encodeVLQ(StringBuffer output, int value) {
+    int signBit = 0;
+    if (value < 0) {
+      signBit = 1;
+      value = -value;
+    }
+    value = (value << 1) | signBit;
+    do {
+      int digit = value & VLQ_BASE_MASK;
+      value >>= VLQ_BASE_SHIFT;
+      if (value > 0) {
+        digit |= VLQ_CONTINUATION_BIT;
+      }
+      output.add(BASE64_DIGITS[digit]);
+    } while (value > 0);
+  }
+}