remove polymer expressions from the repo

pkg/polymer_expressions/CHANGELOG.md

-#### Pub version 0.13.0
-  * Move minimum 'template_binding' constraint to 0.13.0.
-  * Additional polyfills are now required when using this package outside of 
-    polymer. All the files in the 'template_binding' package under lib/js should
-    be included at the top of your app in addition to platform.js from the
-    'web_components' package.
-
-#### Pub version 0.12.0+1
-  * Widen dependency constraint on `observe`.
-
-#### Pub version 0.12.0
-  * Exposed a couple new APIs to create a polymer expression binding outside the
-    context of a template binding.
-  * Updated to depend on latest template_binding and observe. Setting a value on
-    a polymer expression binding now produces a change notification.
-
-#### Pub version 0.11.0
-  * Remove faulty assert that threw when an iterable field was updated.
-  
-#### Pub version 0.11.0
-  * Scopes created by templates are created less often and nested properly. The
-    previous version had a bug with respect to the names visible in 
-    <template releat> tags without an "in" expression, and <template bind> tags.
-    In those templates, names for the outer templates should not be visible.
-    This may result in some breakages in templates that relied on the buggy
-    behavior.
-  * <template bind> now supports "as" expressions.
-  * Removed warnings when attempting to assign a value to a property on null
-    object, or assign a value to a non-assignable expression. Polymer binding
-    initialization sometimes assigns to expressions, so this should reduce
-    unecessary warnings.
-  * Added the % (modulo), === (identical) and !== (not identical) operators.
-  * Fast-path for eval(). eval() no longer observes expressions or creates a
-    tree of observers.
-  * PolymerExpressions bindings clean up expression observers when closed,
-    fixing a potential memory leak.
-  * Better parse errors. Unknown operators are reported, and all exceptions are
-    instances of ParseException so that they can be caught independently of
-    exceptions generated by calling user code.
-
-
-#### Pub version 0.10.0
-  * package:polymer_expressions no longer declares @MirrosUsed. The package uses
-    mirrors at development time, but assumes frameworks like polymer will
-    generate code that replaces the use of mirrors. If you use this directly,
-    you might need to do code generation as well, or add the @MirrorsUsed
-    declaration. This can be done either explicitly or by importing the old
-    settings from 'package:observe/mirrors_used.dart' (which include
-    @reflectable and @observable by default).
-
-  * Errors that occur within bindings are now thrown asycnhronously. We used to
-    trap some errors and report them in a Logger, and we would let other errors
-    halt the rendering process. Now all errors are caught, but they are reported
-    asynchornously so they are visible even when logging is not set up.
-
-  * Fixed several bugs, including:
-      * propagating list changes ([18749][]).
-      * precedence of ternary operators ([17805][]).
-      * two-way bindings ([18410][] and [18792][]).
-
-[17805]: https://code.google.com/p/dart/issues/detail?id=17805
-[18410]: https://code.google.com/p/dart/issues/detail?id=18410
-[18749]: https://code.google.com/p/dart/issues/detail?id=18749
-[18792]: https://code.google.com/p/dart/issues/detail?id=18792