Support generic method syntax in dart2js

[eernstg]

The dart2js compiler should support declaration of type parameters in function and method declarations, and it should support provision of type arguments in message sends. The type parameter declaration grammar should be identical to the grammar of type parameter declarations on classes, and the type argument grammar should be identical to the grammar of type arguments passed to constructors on new expressions.

[eernstg]

https://codereview.chromium.org/1723443003/ provides support for this, when the flag '--enable-generic-methods' is specified. Missing: The specified type parameters are ignored and hence all usages of the type parameters will cause an 'unknown type' warning or possibly a different warning because the same name is declared also in an enclosing scope and the usage is incorrect for that declaration.

[eernstg]

https://codereview.chromium.org/1942763002/ adds support (still behind a flag as mentioned, but it has been renamed to '--generic-method-syntax') for passing method/function type variables into the resolution phase, and making them appear to be the type dynamic. This means that a wide range of usages of the type parameters are now accepted silently (but not type checked: bounds are ignored).

[eernstg]

As of 9bf3b95 (Nov 22, 2016), the generic method syntax feature is enabled by default. The option --generic-method-syntax is still accepted, but has no effect.