Fix some type-checking errors by using different method calls

For example, use `.fetch` or `.dig` instead of `[]`, and use `===` instead of `is_a?` for checking types of objects.

lib/prism/parse_result.rb

@@ -87,9 +87,9 @@ def find_line(byte_offset)
 
       while left <= right
         mid = left + (right - left) / 2
-        return mid if offsets[mid] == byte_offset
+        return mid if (offset = offsets[mid]) == byte_offset
 
-        if offsets[mid] < byte_offset
+        if offset < byte_offset
           left = mid + 1
         else
           right = mid - 1
@@ -262,7 +262,7 @@ def pretty_print(q)
 
     # Returns true if the given other location is equal to this location.
     def ==(other)
-      other.is_a?(Location) &&
+      Location === other &&
         other.start_offset == start_offset &&
         other.end_offset == end_offset
     end
@@ -541,7 +541,7 @@ def pretty_print(q)
 
     # Returns true if the given other token is equal to this token.
     def ==(other)
-      other.is_a?(Token) &&
+      Token === other &&
         other.type == type &&
         other.value == value
     end

lib/prism/parse_result/newlines.rb

@@ -58,6 +58,8 @@ def visit_statements_node(node)
 
     # Walk the tree and mark nodes that are on a new line.
     def mark_newlines!
+      value = self.value
+      raise "This method should only be called on a parse result that contains a node" unless Node === value
       value.accept(Newlines.new(Array.new(1 + source.offsets.size, false))) # steep:ignore
     end
   end

lib/prism/translation/parser/lexer.rb

@@ -302,7 +302,7 @@ def to_a
                 index += 1
               end
             when :tFID
-              if !tokens.empty? && tokens[-1][0] == :kDEF
+              if !tokens.empty? && tokens.dig(-1, 0) == :kDEF
                 type = :tIDENTIFIER
               end
             end