* feat(generators): Add block generator function dictionary
Add a dictionary of block generator functions, provisionally
called .forBlock. Look up generator functions there first, but
fall back to looking up on 'this' (with deprecation notice)
for backwards compatibility.
Also tweak error message generation to use template literal.
* refactor(generators)!: Update generator definitions to use dictionary
* fix(tests): Have blockToCodeTest clean up after itself
Have the blockToCodeTest helper function delete the block generator
functions it adds to generator once the test is done.
* refactor(tests): Use generator dictionary in insertion marker test
The use of generators in insertion_marker_test.js was overlooked
in the earlier commit making such updates, and some test here
were failing due to lack of cleanup in
cleanup in the generator_test.js.
BREAKING CHANGE: this PR moves the generator functions we provide
from their previous location directly on the CodeGenerator instances
to the new .forBlock dictionary on each instance. This does not oblige
external developers to do the same for their custom generators, but
they will need to update any code that references the generator
functions we provide (in generators/*/*, i.e. on javascriptGenerator,
dartGenerator etc.) e.g. to replace the implementation or reuse the
implementation for a different block type.
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