Remove _incrementalModules in DDC compiler and its use in TypeTable

[srujzs]

Historically, we kept around a TypeTable to keep track of types that were used in an expression but never actually imported. This was needed for expression evaluation as it allowed us to import the class. Now that we use the RTI, we no longer need this functionality.

_incrementalModules was used to keep track of these dependencies, but may no longer be needed.

There are guards that check for it e.g.

sdk/pkg/dev_compiler/lib/src/kernel/compiler_new.dart

Line 8020 in 97ab888

if (!_incrementalMode || _incrementalModules.containsKey(module)) {

that can likely just be safely removed. There may be other refactoring opportunities once this is removed e.g. only calling _emitImports if in incremental mode:

sdk/pkg/dev_compiler/lib/src/kernel/compiler_new.dart

Line 3631 in 97ab888

_emitImports(items);

.

cc @nshahan