[ddc] Erase extension types in as expressions

Ensures the same optimizations to expressions statically typed as
extension types.

Issue: #49735
Change-Id: If85f17203cb4c07c06da0e2e1712fea15c39c444
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/342106
Reviewed-by: Mark Zhou <[email protected]>
Commit-Queue: Nicholas Shahan <[email protected]>

Author: nshahan

pkg/dev_compiler/lib/src/kernel/compiler.dart

@@ -7276,7 +7276,7 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
     var jsFrom = _visitExpression(fromExpr);
     if (node.isUnchecked) return jsFrom;
     var to = node.type.extensionTypeErasure;
-    var from = fromExpr.getStaticType(_staticTypeContext);
+    var from = fromExpr.getStaticType(_staticTypeContext).extensionTypeErasure;
 
     // If the check was put here by static analysis to ensure soundness, we
     // can't skip it. For example, one could implement covariant generic caller