dart2js: add initial support for lookup-maps

pkg/lookup_map/README.md

+# Lookup maps
+
+This package contains the definition of `LookupMap`: a simple, but very
+restricted map. The map can only hold constant keys and the only way to use the
+map is to retrieve values with a key you already have.  Expect for lookup, any

[Ivan Posva, 2015/09/08 15:52:10]

Expect or except?

[Siggi Cherem (dart-lang), 2015/09/08 16:00:00]

good catch, thanks!

+other operation in `Map` (like forEach, keys, values, length, etc) is not
+available.
+
+Constant `LookupMap`s are understood by dart2js and can be tree-shaken
+internally: if a key is not used elsewhere in the program, its entry can be
+deleted from the map during compilation without changing the program's behavior.
+Currently dart2js supports tree-shaking keys that are Type literals, and any
+const expression that can only be created with a const constructor. This means
+that primitives, Strings, and constant objects that override the `==` operator
+cannot be tree-shaken.
+
+
+## Examples
+
+`LookupMap` is unlikely going to be useful for individual developers writing
+code by hand. It is mainly intended as a helper utility for frameworks that need
+to autogenerate data and associate it with a type in the program. For example,
+this can be used by a dependency injection system to record how to create
+instances of a given type. A dependency injection framework can store in a
+`LookupMap` all the information it needs for every injectable type in every
+library and package.  When compiling a specific application, dart2js can
+tree-shake the data of types that are not used by the application. Similarly,
+this can also be used by serialization/deserialization packages that can store
+in a `LookupMap` the deserialization logic for a given type.