Enable/tweak web sdk source maps (flutter#13141)

Author: vsmenon

web_sdk/BUILD.gn

@@ -165,15 +165,16 @@ prebuilt_dart_action("flutter_dartdevc_kernel_sdk") {
     "dart:_engine",
     "--no-summarize",
     "--packages",
-    "file:///" + rebase_path("//third_party/dart/.packages"),
+    "file://" + rebase_path("//third_party/dart/.packages"),
     "--multi-root-scheme",
     "org-dartlang-sdk",
     "--multi-root",
     rebase_path("../../"),
     "--multi-root-output-path",
     rebase_path("$root_out_dir/"),
     "--libraries-file",
-    "file:///" + rebase_path("$root_out_dir/flutter_web_sdk/libraries.json"),
+    "file://" + rebase_path("$root_out_dir/flutter_web_sdk/libraries.json"),
+    "--inline-source-map",
     "--modules",
     "amd",
     "-o",