Add a type parameter to TypeAnalyzer to represent type schemas.

The front end and analyzer use the same representation for types and
type schemas, with the unknown type schema (`_`) treated as a
pseudo-type. This creates the risk of accidentally mixing types and
schemas, resulting in `_` accidentally "leaking" into the type system
and showing up in static analysis results or error messages.

As a step toward reducing this risk, this change adds a type parameter
to `TypeAnalyzer`, preventing the shared type analysis code from being
able to assume that types and schemas are represented the same. This
extra discipline makes it much easier to search through the code and
identify how types are manipulated vs. how type schemas are
manipulated, and makes it impossible for the shared type analysis to
accidentally leak `_` into the type system.

I believe this change will also make it easier to implement some type
inference improvements we've been contemplating, such as improved type
inference of `.map(...).toList()`, as well as
dart-lang/language#3471, because those
improvements may require introducing new kinds of type schemas.

Change-Id: Ifcd7e2c4e1172ee39719ce8c8b10d7f10f6a7b6f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/345353
Reviewed-by: Chloe Stefantsova <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/type_inference/type_analysis_result.dart

@@ -288,10 +288,10 @@ class ObjectPatternResult<Type extends Object, Error> {
 }
 
 /// Container for the result of running type analysis on a pattern assignment.
-class PatternAssignmentAnalysisResult<Type extends Object>
-    extends SimpleTypeAnalysisResult<Type> {
+class PatternAssignmentAnalysisResult<Type extends Object,
+    TypeSchema extends Object> extends SimpleTypeAnalysisResult<Type> {
   /// The type schema of the pattern on the left hand size of the assignment.
-  final Type patternSchema;
+  final TypeSchema patternSchema;
 
   PatternAssignmentAnalysisResult({
     required this.patternSchema,
@@ -301,9 +301,10 @@ class PatternAssignmentAnalysisResult<Type extends Object>
 
 /// Container for the result of running type analysis on a pattern variable
 /// declaration.
-class PatternVariableDeclarationAnalysisResult<Type extends Object> {
+class PatternVariableDeclarationAnalysisResult<Type extends Object,
+    TypeSchema extends Object> {
   /// The type schema of the pattern on the left hand size of the declaration.
-  final Type patternSchema;
+  final TypeSchema patternSchema;
 
   /// The type of the initializer expression.
   final Type initializerType;