Refactor how we read global-type-inference data from ssa

tests/compiler/dart2js/arithmetic_simplification_test.dart

Index: tests/compiler/dart2js/arithmetic_simplification_test.dart
diff --git a/tests/compiler/dart2js/arithmetic_simplification_test.dart b/tests/compiler/dart2js/arithmetic_simplification_test.dart
index 039a844d692a2008dbca6bf1b7ee8e5215483b05..a0a0efa9f930f7dd01280f5e142170841cebf75e 100644
--- a/tests/compiler/dart2js/arithmetic_simplification_test.dart
+++ b/tests/compiler/dart2js/arithmetic_simplification_test.dart
@@ -23,10 +23,12 @@ void main() {
 }
 """;
 
+// TODO(johnniwinther): Find out why this doesn't work without the `as num`
+// cast.
 const String NUM_PLUS_ZERO = """
 int foo(x) => x;
 void main() {
-  var x = foo(0);
+  var x = foo(0) as num;
   return x + 0;
 }
 """;
@@ -81,8 +83,7 @@ main() {
   asyncTest(() => Future.wait([
     compileAndDoNotMatch(INT_PLUS_ZERO, 'main', plusZero),
     compileAndDoNotMatch(ZERO_PLUS_INT, 'main', zeroPlus),
-    // TODO(johnniwinther): Find out why this doesn't work without [useMock].
-    compileAndMatch(NUM_PLUS_ZERO, 'main', plusZero, useMock: true),
+    compileAndMatch(NUM_PLUS_ZERO, 'main', plusZero),
     compileAndMatch(ZERO_PLUS_NUM, 'main', zeroPlus),
     compileAndDoNotMatch(INT_TIMES_ONE, 'main', timesOne),
     compileAndDoNotMatch(ONE_TIMES_INT, 'main', oneTimes),