Fix a bug in printing of string literals with escape sequences.

testcases/strong-mode/stringliteral.baseline.txt

Index: testcases/strong-mode/stringliteral.baseline.txt
diff --git a/testcases/strong-mode/stringliteral.baseline.txt b/testcases/strong-mode/stringliteral.baseline.txt
new file mode 100644
index 0000000000000000000000000000000000000000..00fca06a7f28fa9fee631372c94d22f145517d71
--- /dev/null
+++ b/testcases/strong-mode/stringliteral.baseline.txt
@@ -0,0 +1,11 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static field core::String color = "brown";
+static field core::String thing = "lazy dog";
+static field core::String phrase = "The quick ${self::color} fox\njumped over the ${self::thing}.\n";
+static field core::String adjacent = "${self::color}${self::color}${self::color}";
+static field core::String linebreaks = "${self::color}\n${self::color}\n${self::color}";
+static field core::String other = "${self::color}\n is \n${self::color}";
+static method main() → dynamic {}