Allow empty argument list methods.

- Compile them as calls and definitions.
- Use them for call(), clear(), run(), try(), and yield().
- Update the docs.

doc/site/core/fiber.markdown

@@ -13,7 +13,7 @@ fiber is run. Does not immediately start running the fiber.
       IO.print("I won't get printed")
     }
 
-### Fiber.**yield**
+### Fiber.**yield**()
 
 Pauses the current fiber and transfers control to the parent fiber. "Parent"
 here means the last fiber that was started using `call` and not `run`.
@@ -21,37 +21,38 @@ here means the last fiber that was started using `call` and not `run`.
     :::dart
     var fiber = new Fiber {
       IO.print("Before yield")
-      Fiber.yield
+      Fiber.yield()
       IO.print("After yield")
     }
 
-    fiber.call              // "Before yield"
+    fiber.call()            // "Before yield"
     IO.print("After call")  // "After call"
-    fiber.call              // "After yield"
+    fiber.call()            // "After yield"
 
-When resumed, the parent fiber's `call` method returns `null`.
+When resumed, the parent fiber's `call()` method returns `null`.
 
 If a yielded fiber is resumed by calling `call()` or `run()` with an argument,
-`yield` returns that value.
+`yield()` returns that value.
 
     :::dart
     var fiber = new Fiber {
-      IO.print(Fiber.yield) // "value"
+      IO.print(Fiber.yield()) // "value"
     }
 
-    fiber.call          // Run until the first yield.
+    fiber.call()        // Run until the first yield.
     fiber.call("value") // Resume the fiber.
 
-If it was resumed by calling `call` or `run` with no argument, returns `null`.
+If it was resumed by calling `call()` or `run()` with no argument, it returns
+`null`.
 
 It is a runtime error to call this when there is no parent fiber to return to.
 
     :::dart
-    Fiber.yield // ERROR
+    Fiber.yield() // ERROR
 
     new Fiber {
-      Fiber.yield // ERROR
-    }.run
+      Fiber.yield() // ERROR
+    }.run()
 
 ### Fiber.**yield**(value)
 
@@ -63,9 +64,9 @@ Similar to `Fiber.yield` but provides a value to return to the parent fiber's
       Fiber.yield("value")
     }
 
-    IO.print(fiber.call) // "value"
+    IO.print(fiber.call()) // "value"
 
-### **call**
+### **call**()
 
 **TODO**
 
@@ -78,7 +79,7 @@ Similar to `Fiber.yield` but provides a value to return to the parent fiber's
 Whether the fiber's main function has completed and the fiber can no longer be
 run. This returns `false` if the fiber is currently running or has yielded.
 
-### **run**
+### **run**()
 
 **TODO**