Pin down polymer and fix possible widget library bugs

@ussuri 
Pinned down polymer version to match spark ide and added transformer. 

Noticed a some warnings and errors while using `pub serve --mode=debug`. Following patch eliminates those warnings.  

Before:

```
(~/dart/spark/widgets)–($ pub serve --all
Loading source assets... 
Loading smoke/src/default_transformer transformers... (2.2s)
Loading observe transformers... 
Loading polymer/src/build/mirrors_remover transformers... 
Loading polymer transformers... (1.5s)
[Warning from polymer (Linter) on spark_widgets|lib/spark_toolbar/spark_toolbar.html]:
lib/spark_toolbar/spark_toolbar.html:21:1: custom element with name "spark-widget" not found.
[Warning from polymer (Linter) on spark_widgets|lib/spark_tree_view/src/spark_tree_view_node.html]:
lib/spark_tree_view/src/spark_tree_view_node.html:12:1: Invalid url to reach to another package: ../../../packages/spark_widgets/common/spark_widget.html. Path reaching to other packages must first reach up all the way to the packages folder. For example, try changing the url above to: ../../../../packages/spark_widgets/common/spark_widget.html
[Warning from polymer (Linter) on spark_widgets|lib/spark_tree_view/src/spark_tree_view_node.html]:
lib/spark_tree_view/src/spark_tree_view_node.html:14:1: custom element with name "spark-widget" not found.
[Warning from polymer (Linter) on spark_widgets|lib/spark_tree_view/src/spark_tree_view_node.html]:
lib/spark_tree_view/src/spark_tree_view_node.html:32:11: custom element "spark-tree-view-node" extends from "spark-widget", but this tag will not include the default properties of "spark-widget". To fix this, either write this tag as <spark-widget is="spark-tree-view-node"> or remove the "extends" attribute from the custom element declaration.
[Warning from polymer (Linter) on spark_widgets|lib/spark_tree_view/spark_tree_view.html]:
lib/spark_tree_view/src/spark_tree_view_node.html:12:1: Invalid url to reach to another package: ../../../packages/spark_widgets/common/spark_widget.html. Path reaching to other packages must first reach up all the way to the packages folder. For example, try changing the url above to: ../../../../packages/spark_widgets/common/spark_widget.html
[Warning from polymer (Linter) on spark_widgets|lib/spark_tree_view/spark_tree_view.html]:
lib/spark_tree_view/spark_tree_view.html:19:5: custom element "spark-tree-view-node" extends from "spark-widget", but this tag will not include the default properties of "spark-widget". To fix this, either write this tag as <spark-widget is="spark-tree-view-node"> or remove the "extends" attribute from the custom element declaration.
Serving spark_widgets example on http://localhost:8080
Serving spark_widgets test    on http://localhost:8081
[Error from polymer (Linter) on spark_widgets|example/spark_toolbar/example_ui.html]:
example/spark_toolbar/example_ui.html:5:1: couldn't find imported asset "lib/spark_icon_button/spark_icon_button.html" in package "spark_widgets".
[Warning from polymer (Linter) on spark_widgets|example/spark_toolbar/example_ui.html]:
example/spark_toolbar/example_ui.html:9:5: custom element "spark-toolbar" extends from "spark-widget", but this tag will not include the default properties of "spark-widget". To fix this, either write this tag as <spark-widget is="spark-toolbar"> or remove the "extends" attribute from the custom element declaration.
[Warning from polymer (Linter) on spark_widgets|example/spark_toolbar/example_ui.html]:
example/spark_toolbar/example_ui.html:10:7: definition for Polymer element with tag name "spark-icon-button" not found.
Build completed with 1 errors.
```

After: 

```
–(~/dart/spark/widgets)–($ pub serve --all --mode=debug
Loading source assets... 
Loading smoke/src/default_transformer transformers... (2.2s)
Loading observe transformers... 
Loading polymer/src/build/mirrors_remover transformers... 
Loading polymer transformers... 
Serving spark_widgets example on http://localhost:8080
Serving spark_widgets test    on http://localhost:8081
Build completed successfully
```

Also I'm not able to get any of the examples to actually run. The `example/index.html` file does run when using `pub serve --all` but nothing else seems to function correctly. I have not been following polymer close enough to know the hidden bugs or issues. Any advice on how to run the code in the examples folder?