Use "construct" instead of "this" to define constructors.

doc/site/classes.markdown

@@ -172,18 +172,18 @@ constructor, like so:
 
     :::dart
     class Unicorn {
-      this new(name, color) {
+      construct new(name, color) {
         IO.print("My name is " + name + " and I am " + color + ".")
       }
     }
 
-The `this` before the method name makes it a constructor. The `new` isn't
+The `construct` before the method name makes it a constructor. The `new` isn't
 special. Constructors can have any name you like, which lets you clarify how it
 creates the instance:
 
     :::dart
     class Unicorn {
-      this brown(name) {
+      construct brown(name) {
         IO.print("My name is " + name + " and I am brown.")
       }
     }

doc/site/syntax.markdown

@@ -28,8 +28,8 @@ Some people like to see all of the reserved words in a programming language in
 one lump. If you're one of those folks, here you go:
 
     :::dart
-    break class else false for foreign if import in
-    is null return static super this true var while
+    break class construct else false for foreign if import
+    in is null return static super this true var while
 
 ## Identifiers