Fix Flutter beta2 warnings

@@ -58,7 +58,7 @@ class PDFFont extends PDFObject {
   }
 
   PDFRect stringBounds(String s) {
-    var chars = LATIN1.encode(s);
+    var chars = latin1.encode(s);
 
     if (chars.length == 0) return const PDFRect(0.0, 0.0, 0.0, 0.0);
 
@@ -82,7 +82,7 @@ class PDFFont extends PDFObject {
   }
 
   PDFPoint stringSize(String s) {
-    var chars = LATIN1.encode(s);
+    var chars = latin1.encode(s);
 
     var w = 0.0;
     var h = 0.0;

@@ -76,7 +76,7 @@ class PDFStream {
         .replaceAll('\f', '\\f')
         .replaceAll('\r', '\\r');
 
-    putBytes(LATIN1.encode('(' + s + ')'));
+    putBytes(latin1.encode('(' + s + ')'));
   }
 
   static PDFStream text(String s) => new PDFStream()..putText(s);

@@ -51,7 +51,7 @@ void main() {
     for (var i = 10.0; i < 90.0; i += 5.0) {
       g.saveContext();
       var tm = new Matrix4.identity();
-      tm.rotateZ(i * PI / 360.0);
+      tm.rotateZ(i * pi / 360.0);
       tm.translate(300.0, -100.0);
       g.setTransform(tm);
       g.drawString(font1, 12.0, "Hello $i", 20.0, 100.0);