Enable/tweak web sdk source maps

[vsmenon]

This makes SDK source maps / mapping a little more useful by showing dart files instead of dart_sdk.js / dart:sdk_internal.

The file paths are still off as we're copying dart_sdk.js into build_web_compilers to serve it and sources don't actually load in the browser since we're not serving them. We'll need to follow this up with figuring out how to serve properly and fix the paths accordingly.

For now, we'd get the below. Note, the dart:ui URIs show under build_web_compilers per the above:

When the exception was thrown, this was the stack:                      
third_party/dart/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 194:49  throw_
package:build_web_compilers/lib/_engine/engine/surface/picture.dart 284:5                 [_findOrCreateCanvas]
package:build_web_compilers/lib/_engine/engine/surface/picture.dart 257:21                paintCallback
package:build_web_compilers/lib/_engine/engine/surface/surface.dart 71:14                 commitScene
package:build_web_compilers/lib/ui/src/ui/compositing.dart 575:5                          build
package:flutter/src/rendering/layer.dart 727:35                                           buildScene
package:flutter/src/rendering/view.dart 227:36                                            compositeFrame
package:flutter/src/rendering/binding.dart 347:16                                         drawFrame
package:flutter/src/widgets/binding.dart 778:13                                           drawFrame
package:flutter/src/rendering/binding.dart 283:5                                          [_handlePersistentFrameCallback]
package:flutter/src/scheduler/binding.dart 1043:15                                        [_invokeFrameCallback]
package:flutter/src/scheduler/binding.dart 982:9                                          handleDrawFrame
package:flutter/src/scheduler/binding.dart 791:7                                          <fn>
third_party/dart/sdk/lib/_internal/js_dev_runtime/private/isolate_helper.dart 46:17       internalCallback

[vsmenon]

fyi - @ferhatb @yjbanov @jakemac53

[jonahwilliams]

LGTM

[jason-simmons]

@vsmenon It looks like the change to these URLs is breaking the Flutter engine build on Windows. The file:// URLs are being interpreted as paths to network file shares.

(see https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket.appspot.com/8899508523187003088/+/steps/build_host_debug/0/stdout)

[vsmenon]

Thanks for the heads up and revert. I'll take a look.