Format prefix operators. (#1306)

This was an easy one. :)

Author: munificent

lib/src/front_end/ast_node_visitor.dart

@@ -863,7 +863,16 @@ class AstNodeVisitor extends ThrowingAstVisitor<void>
 
   @override
   void visitPrefixExpression(PrefixExpression node) {
-    throw UnimplementedError();
+    token(node.operator);
+
+    // Edge case: put a space after "-" if the operand is "-" or "--" so that
+    // we don't merge the operator tokens.
+    if (node.operand
+        case PrefixExpression(operator: Token(lexeme: '-' || '--'))) {
+      space();
+    }
+
+    visit(node.operand);
   }
 
   @override

test/expression/prefix.stmt

@@ -0,0 +1,41 @@
+40 columns                              |
+>>> Unary negate.
+-  foo  ;
+<<<
+-foo;
+>>> Bitwise not.
+~  foo  ;
+<<<
+~foo;
+>>> Boolean not.
+!  foo  ;
+<<<
+!foo;
+>>> Prefix increment.
+++  foo  ;
+<<<
+++foo;
+>>> Prefix decrement.
+--  foo  ;
+<<<
+--foo;
+>>> Multiple prefix operators.
+-  ~  !  foo;
+<<<
+-~!foo;
+>>> Sequential `-` operators are not joined.
+-  -  -  -foo;
+<<<
+- - - -foo;
+>>> A `-` operator before a negative integer is not joined.
+-  -1;
+<<<
+- -1;
+>>> A `-` operator before a negative floating point number is not joined.
+-  -1.2;
+<<<
+- -1.2;
+>>> A `-` before a `--` is not joined.
+-  --  foo;
+<<<
+- --foo;