Rationalize coercions

test/codegen/expect/sunflower/sunflower.txt

Index: test/codegen/expect/sunflower/sunflower.txt
diff --git a/test/codegen/expect/sunflower/sunflower.txt b/test/codegen/expect/sunflower/sunflower.txt
index 18d2d09ec0f8304a8e255ea65bf93a13d99830bb..c875b966c304b42a8d1513cca65e257262864b4a 100644
--- a/test/codegen/expect/sunflower/sunflower.txt
+++ b/test/codegen/expect/sunflower/sunflower.txt
@@ -1,8 +1,8 @@
 // Messages from compiling sunflower.html
-info: line 20, column 29 of test/codegen/sunflower/sunflower.dart: [DownCast] querySelector("#slider") (Element) will need runtime check to cast to type InputElement
+info: line 20, column 29 of test/codegen/sunflower/sunflower.dart: [AssignmentCast] querySelector("#slider") (Element) will need runtime check to cast to type InputElement
 final InputElement slider = querySelector("#slider");
                             ^^^^^^^^^^^^^^^^^^^^^^^^
-info: line 25, column 5 of test/codegen/sunflower/sunflower.dart: [DownCast] (querySelector("#canvas") as CanvasElement).getContext('2d') (RenderingContext) will need runtime check to cast to type CanvasRenderingContext2D
+info: line 25, column 5 of test/codegen/sunflower/sunflower.dart: [AssignmentCast] (querySelector("#canvas") as CanvasElement).getContext('2d') (RenderingContext) will need runtime check to cast to type CanvasRenderingContext2D
     (querySelector("#canvas") as CanvasElement).getContext('2d');
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 warning: line 28, column 37 of test/codegen/sunflower/sunflower.dart: [ClosureWrapLiteral] (e) => draw() ((dynamic) → void) will need to be wrapped with a closure of type (Event) → void