Inline source maps as part of each script.

pkg/dev_compiler/lib/src/compiler/compiler.dart

Index: pkg/dev_compiler/lib/src/compiler/compiler.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/compiler.dart b/pkg/dev_compiler/lib/src/compiler/compiler.dart
index 75337de7bc9ec6e8929f1708792e258dd2ca6eb1..8758122caa359803875b929d802d6c0388ef6a09 100644
--- a/pkg/dev_compiler/lib/src/compiler/compiler.dart
+++ b/pkg/dev_compiler/lib/src/compiler/compiler.dart
@@ -443,6 +443,15 @@ class JSModuleFile {
   /// the libraries in this module.
   final List<int> summaryBytes;
 
+  /// Unique identifier used to location where to insline the source map

[vsm, 2017/03/13 15:25:28]

insline -> inline

[Jacob, 2017/03/13 15:51:12]

Done.

+  /// in an existing generated JS file
+  ///
+  /// Due to execution order constriants we have cannot generate the source map

[vsm, 2017/03/13 15:25:27]

constriants -> constraints

[Jacob, 2017/03/13 15:51:12]

cleaned up this comment.

+  /// by the time we would need it to insert the sourcemap directly so instead
+  /// we have to generate the JS with this ID and then replace the ID with the
+  /// actual sourcemap after the fact.
+  static String sourceMapHoleID = 'SourceMap3G5a8h6JVhHfdGuDxZr1EF9GQC8y0e6u';
+
   JSModuleFile(
       this.name, this.errors, this.options, this.moduleTree, this.summaryBytes);
 
@@ -485,7 +494,9 @@ class JSModuleFile {
     if (options.sourceMap && sourceMap != null) {
       builtMap =
           placeSourceMap(sourceMap.build(jsUrl), mapUrl, options.bazelMapping);
-
+      if (name == 'dart_sdk') {
+        builtMap = cleanupSdkSourcemap(builtMap);
+      }
       if (options.sourceMapComment) {
         var relativeMapUrl = path
             .toUri(
@@ -493,18 +504,20 @@ class JSModuleFile {
             .toString();
         assert(path.dirname(jsUrl) == path.dirname(mapUrl));
         printer.emit('\n//# sourceMappingURL=');
-        if (options.inlineSourceMap) {
-          var bytes = UTF8.encode(JSON.encode(builtMap));
-          var base64 = BASE64.encode(bytes);
-          printer..emit('data:application/json;base64,')..emit(base64);
-        } else {
-          printer.emit(relativeMapUrl);
-        }
+        printer.emit(relativeMapUrl);
         printer.emit('\n');
       }
     }
 
-    return new JSModuleCode(printer.getText(), builtMap);
+    var text = printer.getText();
+    var rawSourceMap = options.inlineSourceMap ? JSON.encode(builtMap) : null;
+    // Encode the sourcemap as an escaped string rather than JSON
+    // as Dart code using the sourcemap can't take advantage of it being JS
+    // JSON so we might as well encode it as a String which should be quicker
+    // to parse.
+    text = text.replaceFirst(sourceMapHoleID, JSON.encode(rawSourceMap));

[vsm, 2017/03/13 15:25:27]

So, are we sending the source map twice now?  Once as a Dart arg and once
normally?

[Jacob, 2017/03/13 15:51:12]

We are encoding the source map in the source file only once.
I've changed where the sourcemap is inlined with this CL. With the flag set, the
sourceMappingURL still points to the regular url instead of a Data url.
It is true the separate devtools process will request the sourcemaps from a
different url (e.g. yourfile.js.map) with this CL so by that measure the
sourcemap is sent twice.

+
+    return new JSModuleCode(text, builtMap);
   }
 
   /// Similar to [getCode] but immediately writes the resulting files.
@@ -584,3 +597,24 @@ Map placeSourceMap(
   map['file'] = transformUri(map['file']);
   return map;
 }
+
+/// Cleanup the dart_sdk source map.
+///
+/// Strip out files that should not be included in the sdk sourcemap as they
+/// are implementation details that would just confuse users.
+/// Normalize sdk urls to use "dart:" for more understandable stack traces.
+Map cleanupSdkSourcemap(Map sourceMap) {
+  var map = new Map.from(sourceMap);
+  var list = new List.from(map['sources']);
+  map['sources'] = list;
+  for (var i = 0; i < list.length; ++i) {
+    var url = list[i];
+    if (url.contains('/js_lib/')) {

[vsm, 2017/03/13 15:25:28]

Perhaps everything under _internal?

[Jacob, 2017/03/13 15:51:13]

Done.

+      list[i] = null;
+    } else {
+      // TODO(jacobr): use a cleaner way to normalize dart: urls.

[vsm, 2017/03/13 15:25:28]

Yeah, should be able to plumb the analyzer normalized uris through here rather
than mapping back.

What are these paths relative to anyway?  Looks brittle.  

[Jacob, 2017/03/13 15:51:12]

I think these relative urls are technically correct given where the dart_sdk.js
is generated. I've removed this line from the CL. The right place to do the
normalization to dart:html urls is probably really when we parse the Dart stack
traces. There is existing support for that although I wasn't able to get it
working Friday.

+      list[i] = url.replaceFirst('../../../gen/patched_sdk/lib/', 'dart:');
+    }
+  }
+  return map;
+}