Remove ContextBuilder and its tests

[scheglov]

Instead we want to create AnalysisContextCollection and verify the created analysis contexts, with files, environment, etc.
As you can imagine, my preference is to dump all this state as text, to get comprehensive view easily.
Kind of like this.

sdk/pkg/analyzer/test/src/dart/analysis/file_state_test.dart

Lines 3035 to 3057 in 451d919

	test_newFile_noDirectives() async {
	final a = newFile('$testPackageLibPath/a.dart', '');
	fileStateFor(a);
	assertDriverStateString(testFile, r'''
	files
	/home/test/lib/a.dart
	uri: package:test/a.dart
	current
	id: file_0
	kind: library_0
	libraryImports
	library_1 dart:core synthetic
	cycle_0
	dependencies: dart:core
	libraries: library_0
	apiSignature_0
	unlinkedKey: k00
	libraryCycles
	elementFactory
	''');
	}

ContextBuilder should not be used by clients, which should be migrated to AnalysisContextCollection.

[pq]

FWIW I'm strongly in favor of not text. Otherwise, I don't have enough context.