Implement string interpolation up to SSA.

dart/frog/leg/typechecker.dart

Index: dart/frog/leg/typechecker.dart
diff --git a/dart/frog/leg/typechecker.dart b/dart/frog/leg/typechecker.dart
index bd0d8d11bc90eb762b05359e9d9f02050b7bdfe0..e0b95b8ad0ba0fd3d1b24d1228778a22e201b400 100644
--- a/dart/frog/leg/typechecker.dart
+++ b/dart/frog/leg/typechecker.dart
@@ -532,4 +532,13 @@ class TypeCheckerVisitor implements Visitor<Type> {
       return objectType;
     }
   }
+
+  visitStringInterpolation(StringInterpolation node) {
+    node.visitChildren(this);
+    return stringType;
+  }
+
+  visitStringInterpolationPart(StringInterpolationPart node) {
+    node.visitChildren(this);
+  }
 }