Support multiple partial calls from a single template

[srawlins]

There was a bug in the partial-compiler-generating code that cached the results of compiling one partial, keyed only to the partial path. But a partial has a few input parameters that could differ. For example, given two classes, A and B that each extend Base, and a template that iterates over some A values and some B values, going into a partial for each (see the test case in the PR). The first time the partial is encountered, the compiler would create a partial compiler with an A context type. And the second time it is encountered, it would use the cached compiler, seeing the same partial path. But this results in passing B values into the partial renderer function where A values are expected.

The fix is to instead create multiple compilers for this partial, and let the existing deduplication code collapse them back into one. So a partial compiler for the A context type is created, and one is created for the B context type (and each compiler is used to create a renderer function). Later, the deduplication code finds a good LUB for A and B and merges the two renderer functions into one.

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[srawlins]

Ready for review, @dart-lang/analyzer-team